This invention relates to an apparatus for injecting treatment chemicals into an oil producing well. In one aspect it relates to an apparatus that can be used with a subsurface pump, preferably a subsurface rotary pump, used to pump the crude oil to the surface during, production. In another aspect it relates to an injector apparatus with no moving parts that may be used in lieu of a surface chemical pump.
In the production phase of an oil well, it is usually necessary to artificially lift the crude oil from its natural level in the wellbore to the wellhead. The two most common lift methods are to use either a surface pumping unit or a subsurface rotary pump. A familiar sight in the oil fields around the world is the horse head bobbin up and down on a conventional beam pumping unit (pump jack). This method of bringing oil to the surface accounts for between 70% to 80% of the artificial lifting of oil. The pumping unit may be powered by either an electric motor or an internal combustion engine. In either case it is usually necessary to couple the motor and pump through a speed reducer. A reduction of 30 to 1 is typically needed to operate the pump at 20 strokes per minute (spm). The rotation of the prime mover is converted into an up-and-down motion of the beam and horse head through a pitman/crank assembly. The oscillating horse head of the pumping unit raises and lowers a sucker rod and reciprocates the sucker rod pump in the wellbore. This action lifts the oil on the upstroke to the wellhead. Because these pumps operate at low speed the average pumping rate in barrels per day (B/D) tends to be relatively low. However, the flow rate on the upstroke is much higher than the average rate, and in many instances can be sufficient for purposes of the present invention.
An electrically-powered subsurface pump consists essentially of a rotary centrifugal pump with the shaft directly coupled to an electric motor. The entire unit is cylindrical and is sized to fit inside the well casing. It is connected to the well tubing (i.e. central flow line) and has an insulated electrical cable attached to the outside of the tubing. The submersible equipment and cable are lowered into the well as the tubing is being un in on the surface. The pressure created by the rotation of the pump""s impellers forces the fluid to the surface through the tubing. Because the pump runs at the same speed as the motor, submersible electrical pumps are capable of pumping larger volumes of fluids than conventional surface beam pumping units. For this reason, submersible pumps are often used where the oil-to-water ratio is high. A typical submersible rotary pump may lift from 250 to 26,000 B/D depending on the size of the casing and the depth of the well.
During production it is often necessary to inject a treatment chemical into the annular space between the well casing and tubing. These might include demulsifiers, corrosion inhibitors, scale inhibitors, paraffin inhibitors, etc. Demulsifiers are chemicals used to dehydrate crude oil containing emulsified water. In many cases this water-in-oil emulsion is very stable. Without the use of a demulsifier, the water would not separate from the crude oil. The rapid separation of the water from the oil phase may be necessary at the well site because of limited storage capacity. The combined total of water remaining in the crude oil must be below 1% in most cases. Excess water can cause serious corrosion problems in pipelines and storage tanks. In addition, water in a refinery stream can interfere with the distillation process and damage the refinery equipment.
In wells which use a production pumping unit, a small chemical pump may be used to inject the treatment chemical into the wellhead. The chemical pump may be powered by the same up-and-down movement of the pumping unit using a connecting rod. Several types of chemical pumps are known in the art. Although these mechanically actuated pumps are widely used they nevertheless present problems due to mechanical failure and plugging
In a well using a submersible rotary pump, generally the only source of power at the surface is electrical. Using an electrically powered chemical surface pump has been found to be uneconomical because of the cost of transformers and other electrical equipment required to power the pump.
The present invention provides an apparatus for injecting treatment chemicals into an oil well. The injector apparatus is particularly adapted for use in wells being produced using a submersible rotary pump, but can also be used with reciprocating sucker-rod pumps. The present injector may be used in lieu of an electrical or mechanical surface chemical pump. The apparatus requires no power input and therefore is economical to operate. Because the apparatus has no moving parts it is reliable and easy to maintain.
The present apparatus uses a venturi flow nozzle to create a vacuum pressure source in a pipe to draw (suck) a treatment chemical from a chemical storage tank into the pipe. The pipe is connected to the wellhead and the chemical flows into the annular space between the well casing and the tubing for treating the well.
A submersible pump located near the bottom of the well draws fluid (crude oil) from the annular space between the casino and tubing and lifts the fluid through the tubing to the wellhead. At the wellhead a flow line conducts the produced fluid to a separation vessel storage tank or other collection means. The injector apparatus of the present invention is mounted in a small pipe or tubing (side stream or by-pass line) which interconnects the wellhead and the annulus. Thus a portion of the produced fluid is collected as in usual production, and a portion flows into the injector. The injector apparatus comprises a venturi nozzle having a throat of reduced flow area. Basic physics requires that the produced fluid flowing into the nozzle accelerate in the throat thereby increasing the kinetic energy of the flow. The increase in kinetic energy comes at the expense of the pressure energy (also called flow energy) and as a result the pressure in the throat decreases and a suction pressure is created in the nozzle. A flow line interconnects the throat of the venturi with a storage tank containing a treatment chemical to be injected into the well. Because the pressure in the throat is less than in the tank, the chemical flows from the tank into the nozzle and mixes with the fluid in the by-pass line and discharges into the annulus. The amount of chemical injected into the annulus may be adjusted by controlling the flow rate of fluid in the by-pass line. The flow rate of chemical in the line interconnecting the injector apparatus and the chemical storage tank is controlled with adjustable valves.
The submersible pump located near the bottom of the well operates continuously and acts to mix the fluid in the well annulus and tubing whereby the injected chemical is dispersed throughout the fluid in the entire well thereby treating the well. The flow rate of chemical into the well is controlled using adjustable valves in the injector apparatus. A wide range of flow rates are possible depending on the production flow rate of produced fluid.